What does this Mizmor discuss?
Radak: It is about Yetzi'as Egypt. It mentions ceasing the servitude of our ancestors, on Rosh Hashanah - "Hasirosi mi'Sevel Shichmo" (verse 7). Therefore, this Mizmor was said in the Beis ha'Mikdash on Rosh Hashanah 1 . It also mentions Teki'as Shofar (verse 4).
Malbim: It was established on Rosh Hashanah. It seems that it was a time of famine, and on Rosh Hashanah they prayed for rain, and they were answered.
And so it is Shir Shel Yom for Rosh Hashanah, based on Minhag ha'Gra, and also for Thursday, and for Hoshana Raba (unless it falls on Friday - then it is said the day before). (PF)
What is "ha'Gitis"?
Rashi: It is a musical instrument.
